In recent years, a great effort has been exerted in exploring for and producing oil from oil fields under water. The Gulf of Mexico and the North Sea are specific examples where a great effort has been exerted.
Many techniques have been explored for efficient exploration and production of these undersea oil reserves. One recent development is the tension leg platform which can be used both for drilling and production. The tension leg platform (commonly referred to as TLP) is a floating structure, resembling a large semisubmersible drilling rig, connected to sea bed foundation templates by vertical mooring tethers. Buoyancy for the TLP is provided by watertight columns, pontoons and the like. The TLP is provided with an excess of buoyancy to keep the mooring tethers in tension for all weather and loading conditions.
Three separate marine riser systems are commonly used for conducting fluids between the subsea template and the TLP during both drilling and production phases. These riser systems are the drilling, production and crude oil sales risers. The risers are secured at the sea floor on the subsea template and extend to the TLP. The risers must be maintained constantly in tension to avoid the risers collapsing from their own weight, despite movement of the TLP due to surface movement and weather extremes.
In the past, active hydropneumatic systems have been used to maintain a tension on the risers in TLP systems. Such use is described in a paper entitled "Conoco TLP Riser Tensioning Systems" authored by M. H. Frayne and F. L. Hettinger. Tensioners disclosed in this reference incorporate hydraulic actuators which stroke up and down in response to TLP movements to apply a relatively constant tension to each riser. This system has several disadvantages. It is an active system which requires continuous supply of high pressure fluids for operation. Thus, if a malfunction occurs which eliminates the supply of this high pressure fluid, the system can fail. Further, a sophisticated and expensive control system must be provided which maintains the desired pressure in the system. Therefore, a need exists for an improved tensioner system which avoids these disadvantages.